1. Field of the Invention
The present invention is directed to a novel synthetic method for producing nanoscale heterostructures, and particularly nanoscale heterostructures that comprise a metal core and a monocrystalline semiconductor shell with substantial lattice mismatches between them. More specifically, the invention concerns the use of controlled soft acid-base coordination reactions between molecular complexes and colloidal nanostructures to drive the nanoscale monocrystalline growth of the semiconductor shell with a lattice structure incommensurate with that of the core. The invention also relates to more complex hybrid core-shell structures that exhibit azimuthal and radial nano-tailoring of structures. The invention is additionally directed to the use of such compositions in semiconductor devices.
2. Description of Related Art
Semiconductor-metal hybrid heterostructures are promising building blocks for applications in catalytic, magnetic, and opto-electronic devices (Maynor, B. W. et al. (2004) “Site-Specific Fabrication of Nanoscale Heterostructures: Local Chemical Modification of GaN Nanowires Using Electrochemical Dip-Pen Nanolithography,” J. Am. Chem. Soc. 126:6409-6413; Alemseghed, M. G. et al. (2011) “Controlled Fabrication of Colloidal Semiconductor-Metal Hybrid Heterostructures: Site Selective Metal Photo Deposition,” Chem. Mater. 23:3571-3579). The semiconductor's tunable band gap (300-4000 nm 4.1-0.3 eV) broad and intense absorption (ε≈105-106 L mol−1 cm−1), and long-lived exciton (up to 40 ns for CdSe, 1.8 μs for PbS) provide unmatched light absorption and emission capabilities. The metal can serve as an additional chromophore, fluorescence enhancer, paramagnet, or charge-collecting material where carriers localize after exciton quenching. For example, semiconductor-metal hybrid heterostructures have been shown to convert solar energy into potential and chemical energy. They become redox-active upon illumination and remain redox-active after being stored in the dark for several hours. Thus, such structures have utility in a variety of applications including: field-effect transistors (Yoshida, S. et al. (1998) “Reliability Of Metal Semiconductor Field-Effect Transistor Using Gan At High Temperature,” J. Appl. Phys. 84 (5):2940-2942; Wu, Y. et al. (2004) “Single-Crystal Metallic Nanowires And Metal/Semiconductor Nanowire Heterostructures,” Nature 430(6995):61-65), photodetectors, photodiodes (Endo, H. et al. (2007) “Schottky Ultraviolet Photodiode Using A ZnO Hydrothermally Grown Single Crystal Substrate,” Appl. Phys. Lett. 90(12):121906-121908), solar cells (Chandrasekharan, N. et al. (2000) “Improving the Photoelectrochemical Performance of Nanostructured TiO2 Films by Adsorption of Gold Nanoparticles,” J. Phys. Chem. B 104(46):10851-10857; Nakato, Y. et al. (2002) “Effect Of Microscopic Discontinuity Of Metal Overlayers On The Photovoltages In Metal-Coated Semiconductor-Liquid Junction Photoelectrochemical Cells For Efficient Solar Energy Conversion,” J. Phys. Chem. 92 (8):2316-2324), catalysis (Valden, M. et al. (1998) “Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties,” Science 281(5383):1647-1650: Subramanian, V. et al. (2004) “Catalysis with TiO2/Gold Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration,” J. Am. Chem. Soc. 126(15):4943-4950; Hirakawa, T. et al. (2005) “Charge Separation and Catalytic Activity of Ag@TiO2 Core—Shell Composite Clusters under UV-Irradiation,” J. Am. Chem. Soc. 127(11):3928-3934), nanodevice wiring (Lu, W. et al. (2007) “Nanoelectronics From The Bottom Up,” Nat. Mater. 6(11):841-850; McAlpine, M. C. et al. (2007) “Highly Ordered Nanowire Arrays On Plastic Substrates For Ultrasensitive Flexible Chemical Sensors,” Nat. Mater. 6:379-384; Cui, Y. et al. (2001) “Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species,” Science 293(5533):1289-1292) and sensing (see, Kundu, P. et al. (epub 15 Dec. 2009) “Nanoscale Heterostructures with Molecular-Scale Single-Crystal Metal Wires,” J. Am. Chem. Soc. 132:20-21).
In particular, metal nanoparticles (NPs) having sizes comparable to their electron mean free path possess unusual properties and functionalities (Klimov, V. I. SEMICONDUCTOR AND METAL NANOCRYSTALS: SYNTHESIS AND ELECTRONIC AND OPTICAL PROPERTIES (Marcel Dekker, New York (2003)), serving as model systems to explore quantum and classical coupling interactions as well as providing building blocks for practical applications (Tang, Y. et al. (epub 19 Aug. 2007) “Tailoring Properties And Functionalities Of Metal Nanoparticles Through Crystallinity Engineering,” Nature 6:754-759). Such applications include: quantum dot analysis (Collier, C. P. et al. (1997) “Reversible tuning of silver quantum dot monolayers through the metal-insulator transition,” Science 277:1978-1981); catalysis (Valden, M. et al. (1998) “Onset Of Catalytic Activity Of Gold Clusters On Titania With The Appearance Of Nonmetallic Properties,” Science 281:1647-1650; Zheng, X. et al. (2009) “Nickel/Nickel Phosphide Core-Shell Structured Nanoparticles: Synthesis, Chemical, and Magnetic Architecture,” Chem. Mater. 21:4839-4845); nucleic acid detection (Cao, Y. C. (2002) “Nanoparticles With Raman Spectroscopic Fingerprints For DNA And RNA Detection,” Science 297:1536-1540); assessing changes in single biomolecules (Sönnichsen, C. et al. (2005) “A Molecular Ruler Based On Plasmon Coupling Of Single Gold And Silver Nanoparticles,” Nature Biotechnol. 23:741-745 (2005); Nie, S. et al. (1997) “Probing Single Molecules And Single Nanoparticles By Surface Enhanced Raman Scattering,” Science 275:1102-1106) and photonic devices (Sherry, L. J. et al. (2006) “Localized Surface Plasmon Resonance Spectroscopy Of Single Silver Triangular Nanoprisms,” Nano Lett. 6:2060-2065); Maier, S. A. et al. (2003) “Local Detection Of Electromagnetic Energy Transport Below The Diffraction Limit In Metal Nanoparticle Plasmon Waveguides,” Nature Mater. 2:229-232).
Although advances in strategies for synthesizing metal NPs have enabled control of size, composition and shape (Zheng, N. et al. (2006) “One-Step One-Phase Synthesis Of Monodisperse Noble-Metallic Nanoparticles And Their Colloidal Crystals,” J. Am. Chem. Soc. 128:6550-6551; Jin, R. et al. (2003) “Controlling Anisotropic Nanoparticle Growth Through Plasmon Excitation,” Nature 425, 487-490; Sun, Y. et al. (2002) “Shape-Controlled Synthesis Of Gold And Silver Nanoparticles,” Science 298:2176-2179; Murray, C. B. et al. (2000) “Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies,” Annu Rev. Mater. Sci. 30, 545-610; Sun, S. et al. (2000) “Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices,” Science 287:1989-1992), the requirement that defects are simultaneously controlled, to ensure essential perfect nanocrystallinity for physics modeling as well as device optimization, is a potentially more significant issue, but has posed substantial technological challenges.
Thus, an ability to precisely control the growth of single-crystal semiconductor-based heterostructures with modulated composition is considered to be a prerequisite for exploring fundamental nanoscale semiconductor physics (Ayers, J. E. (2007) HETEROEPITAXY OF SEMICONDUCTORS: THEORY, GROWTH AND CHARACTERIZATION(CRC Press, New York); Ryzhii, M. et al. (2008) PHYSICS AND MODELING OF TERA-AND NANO-DEVICES (World Scientific, Singapore, 2008) and can offer technological devices with optimum characteristics, including enhanced optical properties with high quantum yields (McBride, J. et al. (2006) “Structural Basis for Near Unity Quantum Yield Core/Shell Nanostructures,” Nano Lett. 697):1496-1501), engineered electronic bandgaps (Battaglia, D. et al. (2003) “Colloidal Two-Dimensional Systems: CdSe Quantum Shells and Wells,” Angew. Chem. Int. Ed. 42(41):5035-5039 (2003); Kim, S. et al. (2003) “Type-II Quantum Dots: CdTe/CdSe(Core/Shell) and CdSe/ZnTe(Core/Shell) Heterostructures,” J. Am. Chem. Soc. 125 (38): 11466-11467; Smith, A. M. et al. (2010) “Semiconductor Nanocrystals: Structure, Properties, and Band Gap Engineering,” Acc. Chem. Res. 43(2):190-200), and various solid-state optoelectronic properties (Klimov, V. I. et al. (2007) “Single-Exciton Optical Gain In Semiconductor Nanocrystals,” Nature 447(7143):441-446; Caruge, J. M. et al. (2008) “Colloidal Quantum-Dot Light-Emitting Diodes With Metal-Oxide Charge Transport Layers,” Nat. Photonics 2(4):247-250; E. H. Sargent (2009) “Infrared Photovoltaics Made By Solution Processing,” Nat. Photonics 3(6):325-331 (2009). Unintentional crystalline imperfections (such as polycrystallinity, dislocations, and other structural defects) lead to performance degradation or even premature failure of devices. For example, although the optical quality of semiconductor CdSe nanoparticles (NPs) could be improved by an overlayer of epitaxially grown CdS or ZnS, problems appear once the shell thickness becomes larger than the “critical” layer thickness (about two monolayers) due to the existence of strain-induced defects (McBride, J. et al. (2006) “Structural Basis for Near Unity Quantum Yield Core/Shell Nanostructures,” Nano Lett. 697):1496-1501; Chen, X. B. et al. (2003) “Coherency Strain Effects on the Optical Response of Core/Shell Heteronanostructures,” Nano Lett. 3(6):799-803; Peng, X. G. et al. (1997) “Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility,” J. Am. Chem. Soc. 119(30):7019-7029). Current methods that achieve high-quality monocrystalline heterostructures are all based on epitaxial growth, which requires moderate lattice mismatches (<2% ) between the two different materials. Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where the overlayer is in registry with the substrate. Such a lattice-matching constraint is a severe obstacle, particularly for growth of core-shell nanostructures with (quasi-) spherical core NPs with highly curved surfaces that present many different crystallographic facets (Wang, Z. L. (2000) “Transmission Electron Microscopy of Shape-Controlled Nanocrystals and Their Assemblies,” J. Phys. Chem. B 104(6):1153-1175). In addition to such lattice-matching requirements, the issues related to differences in crystal structure, bonding, and other properties have been found to inhibit epitaxial growth of dissimilar hybrid materials such as monocrystalline semiconductors on metals (Palmstrøm, C. J. (1995) “Epitaxy of Dissimilar Materials,” Annu Rev. Mater. Sci. 25:389-415).
Attempts to use epitaxy to achieve hybrid core-shell nanostructures have been unsuccessful, resulting in either polycrystalline semiconductor shells or anisotropic structures with segregation of the core and shell, thus limiting their usefulness (Lee, J. S. (2008) “Au-PbS Core-Shell Nanocrystals: Plasmonic Absorption Enhancement and Electrical Doping via Intra particle Charge Transfer,” J. Am. Chem. Soc. 130(30):9673-9675; Kim, H. et al. (2005) “Synthesis and Characterization of Co/CdSe Core/Shell Nanocomposites: Bifunctional Magnetic-Optical Nanocrystals,” J. Am. Chem. Soc. 127(2):544-546; Mokari, T. et al. (2005) “Formation Of Asymmetric One-Sided Metal-Tipped Semiconductor Nanocrystal Dots And Rods,” Nat. Mater. 4(11):855-863 (2005); Mokari, T. et al. (2004) “Selective Growth of Metal Tips onto Semiconductor Quantum Rods and Tetrapods,” Science 304(5678):1787-1790; Wang, C. et al. (2009) “Recent Progress in Syntheses and Applications of Dumbbell-like Nanoparticles,” Adv. Mater. 21(30):3045-3052; see also, Zhang, J. et al. (2009) “Versatile Strategy for Precisely Tailored Core@Shell Nanostructures with Single Shell Layer Accuracy: The Case of Metallic Shell,” Nano letters 9(12):4061-4065).
Thus, a need exists for improved methods capable of achieving hybrid core-shell nanostructures. In particular, a need exists for a general non-epitaxial growth strategy capable of providing precise control over the formation of the hybrid core-shell nanostructures, so as to permit the production of hybrid core-shell nanostructures whose monocrystalline semiconductor shells are not dependent on the structure of the core nanoparticle (NP). The present invention is directed to this and related needs.